Changes in nonextreme precipitation may have not-so-subtle consequences

CHAMPAIGN, Ill. — Major floods and droughts receive a lot of attention in the context of climate change, but University of Illinois researchers analyzed over five decades of precipitation data from North America to find that changes in nonextreme precipitation are more significant than previously realized and larger than those in extreme precipitation. These changes can have a strong effect on ecosystems, agriculture, infrastructure design and resource management, and point to a need to examine precipitation in a more nuanced, multifaceted way.

"This study articulates how everyday precipitation events — not just the extremes that have been the focus of most studies — are changing," said Illinois civil and environmental engineering professor and lead author Praveen Kumar . "It's not just the amount of rainfall that is important; it's the duration of that rainfall and the amount of time between rainfalls and dry periods."

The study, published in Nature Scientific Reports, is the most comprehensive of its type, said graduate student and co-author Susana Roque-Malo.

"We used data from more than 3,000 weather stations across North America," Roque-Malo said. "There are a few other studies that use a similar methodology, but they have focused on smaller sections of the continent or parts of Europe."

The researchers identified several regions where the microclimates – the local climate determined by elevation and ecosystem – appear to have a significant effect on local and regional precipitation trends.

In Oregon's Willamette Valley, the researchers observed decreases in the total annual precipitation, the number of days per year with precipitation and the number of consecutive days with precipitation. The areas immediately surrounding the valley, however, experienced increases in those measures.

"Examples like this indicate that changes in precipitation patterns are much more nuanced, and that it may not be the best practice to make broad assumptions like 'all wet areas are becoming wetter and dry areas are becoming drier,' as many climate change discussions assert," Roque-Malo said.

These observations have important implications on the resilience of ecosystems, agriculture and water resource planning, the researchers said.

"Successive generations of ecosystems evolve through adaptation to climatic change," Kumar said. "If that rate of change, however small, exceeds the adaptive capacity, these environments will become susceptible to collapse."

"Hydroelectric plants, storm water drainage systems – any structure that relies on an assumption of expected precipitation – could be vulnerable as we look toward becoming more climate-resilient," Roque-Malo said.

Although current climate models may not be able to resolve these types of small but creeping changes observed in this study, the researchers hope that this work will inform and provide validation criteria for more sophisticated future models and assessment of the impact of climate change.

"This study confirms that there is more to climate than the number and size of extreme events," says Richard Yuretich, a program director in the National Science Foundation's Division of Earth Sciences, which funded the research. "Shifts in the daily pattern of rainfall, sometimes subtle, also occur. These can be very hard to document, but the existence of long-term monitoring sites provides the information needed to recognize trends and plan for future changes."

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The National Science Foundation's Intensively Managed Landscapes Critical Zone Observatory project, the University of Illinois Graduate College Fellowship and SURGE Fellowship supported this research.